Aluminum castings are lightweight and easily moldable and therefore have been used in various fields such as architectural members, decorative items, automobile and motorcycle parts and automobile wheels. They are, however, destined to undergo the problem of corrosion if the metal materials are used as they are. As such, coating is often carried out in order to secure corrosion resistance for finished products and to improve appearances of the finished products. When coating is carried out directly on the metal products, however, coating adhesion and/or corrosion resistance after coating as required in the market may not be satisfied. Therefore, surface treatment is performed before carrying out coating.
As such surface treatment, chromate treatment process, for example, is a treatment process which provides excellent adhesion and corrosion resistance. However, hexavalent chromium used for such chromate treatment has traditionally been noted with its carcinogenicity. Therefore, in case of reaction type chromate treatment, such measures are taken as closing a rinsing step after chromate treatment. Also, application type chromate treatment has been in practical use, where drying is carried out instead of rinsing after chromate treatment. Any type of chromate treatment tends to be avoided, however, because coated films contain hexavalent chromium so that the possibility of chromium eluting from the films may not completely be eliminated and because of the problem in operational safety and health environment at the time of surface treatment.
Patent Reference 1: Japanese Unexamined Patent Publication 2007-107069
Patent Reference 2: WO2003/074761
Patent Reference 3: Japanese Unexamined Patent Publication 2004-76024
Patent Reference 4: Japanese Unexamined Patent Publication Hei 7-188956
Patent Reference 5: Japanese Unexamined Patent Publication Hei 11-181587
Patent Reference 6: Japanese Unexamined Patent Publication 2006-2229
Then, surface treatment of aluminum castings through nonchromate conversion treatment not containing hexavalent chromium may be contemplated. When aluminum castings are treated before coating through such nonchromate conversion treatment, however, sufficient coating adhesion or corrosion resistance after coating may not be obtained. For aluminum wheels in particular, in addition to heavy surface fouling due to casting and a wide variation in degree of such fouling, coating adhesion and/or corrosion resistance after coating are likely to decline even when shot blasting is carried out, because shot powders adhere to the aluminum wheels. Therefore, means for securing their stable performances have been strongly hoped for.
Patent Reference 1 describes that, prior to nonchromate treatment of aluminum-based substrates, treating with a strong alkaline aqueous solution or alkaline electrolyzed water enables to improve coating adhesion and/or corrosion resistance after coating. According to the experiments carried out by the present inventors, however, it was found that defective treated surface appearances or defective film performances after coating may arise in thus treated aluminum castings. As such, the present invention has a primary object of providing a means for avoiding defective treated surface appearances or defective film performances after coating in techniques for non-chromate conversion treatment of aluminum castings even with a surface conditioning step using a strong alkali before the conversion treatment.
Further, the present inventors have found that continued use of the strong alkali solution of Patent Reference 1 caused variability in treated surface appearances and/or film performances after coating among aluminum castings. As such, the present invention has an object of providing a means for avoiding variability in treated surface appearances and/or film performances among aluminum castings in production lines.
As a result of conducting a prior art search in relation to the present invention, the applicants have found, in addition to the above Patent Reference 1, Patent References 2 to 6 to be described below as being relevant to the present invention. However, each of the techniques described in the Patent References has a problem such that it is impossible to improve coating adhesion and corrosion resistance after coating when aluminum castings are coated or that material surfaces need to be sufficiently cleaned. Detailed discussion will be made below.
First, Patent Reference 2 discloses a surface treatment composition, a surface treatment liquid and a process for surface treatment to be used for depositing surface treatment films excellent in corrosion resistance without coating and corrosion resistance after coating on the surfaces of aluminum, aluminum alloys, magnesium or magnesium alloys using a treatment liquid which will not release wastes such as sludge and does not contain ingredients hazardous to the environment such as hexavalent chromium, and such metal materials excellent in corrosion resistance without coating.
Patent Reference 3 discloses a process for conversion treatment with a conversion treatment agent consisting of zirconium and/or titanium ions, fluorine ions, phosphate ions and a soluble epoxy resin, excellent in corrosion resistance and adhesion, and suitably applicable to aluminum substrates such as aluminum die castings AC-4C and AC-4-CH, and 5000 series and 6000 series aluminum alloys, without using chromium.
According to these inventions, it is understood that adhesion and corrosion resistance after coating of aluminum-based materials can be improved. According to the working examples of these inventions, however, as a precondition for the conversion treatment processes, the aluminum-based materials need cleaning through a degreasing step or the like. When the material surface happens to be uncleaned due to some anomaly on the surface, it is unlikely that targeted coating performances can be provided. Further, in addition to the anomaly on the material surface, when a degreasing agent or the like suffers reduced performances due to aging of the degreasing agent etc., it is unlikely that targeted film coating performances can be accomplished.
Patent Reference 4 discloses a process for surface treatment of Mg containing aluminum alloy sheets for automobile bodies, characterized by washing treatment in a weakly alkaline alkali solution at pH 12 or lower in surface treatment before application of a low viscosity oil for press molding the Mg containing aluminum alloy sheets for automobile bodies. This invention alleges that, uniformity of zinc phosphate films can be improved without decreasing moldability in the case when a low viscosity oil is applied by washing treatment for from 10 to 120 s using an alkali solution of which pH is from 9 to 12 and etching rate is 4 g/m2 or less when immersed in the alkali solution for 300 s. This invention relates to, however, a procedure for the purpose of improving uniformity of zinc phosphate films over the surface of aluminum sheet materials after press working and differs from the present invention in its objects, materials to be treated and treatment steps.
Patent Reference 5 discloses an alkaline liquid detergent for metals storable at room temperature, excellent in detergency and liquid stability with age, which contains a concentrated alkali component but contains a concentrated detersive active ingredient equal to or higher than the solubility in a uniformly emulsified and dispersed condition. With this method, room temperature storable, one-pack type detergents having detergency equal to or higher than that of conventional products can be produced at a low cost. This invention relates to, however, a procedure for producing stable, one-pack type detergents and differs from the present invention in its objects, materials to be treated, treatment steps and treatment methods.
Patent Reference 6 discloses a detergent exhibiting high etching performance on aluminum and its alloys in a very short time. This invention alleges that it is possible to provide excellent detergency with an alkali metal, a maleic acid copolymer and a hydroxyl-carboxylic acid. This invention relates to, however, a procedure for the purpose of exhibiting high etching performance in a very short time, maintaining the high etching performance at relatively low pH ranges compared to conventional products, increasing the allowable amount of aluminum ions and keeping the generated sludge level at a low level, and differs from the present invention in its objects and treatment steps.